1. Field of the Invention
The present invention relates to driver circuits and, more particularly, to a driver on an integrated circuit that limits the voltage of a wave front launched onto a transmission line.
2. Description of the Related Art
Digital chips on a printed circuit board communicate with each other by sending and receiving streams of data across conductive traces, known as interconnects, which electrically connect the chips together. Common examples of interconnects include strip lines, microstrips, and cables which have conductors. A stream of data includes a sequence of logic highs and logic lows. A logic high is typically represented by a high voltage, while a logic low is typically represented by ground.
Recently, the time required for a signal to transition from a logic low to a logic high, known as the rise time, and the time required for a signal to transition from a logic high to a logic low, known as the fall time, has been significantly reduced. These reductions in rise and fall times, however, have significantly changed the way that the interconnects operate when transferring data from one chip to another chip.
Historically, the rise and fall times were relatively long with respect to the delay time required for a signal to propagate along an interconnect from one chip to another chip. As a result, an interconnect was typically modeled as a lumped capacitive load. More recently, however, with the reduced rise and fall times, the interconnects no longer respond as lumped capacitive loads, but instead now respond as transmission lines.
When a driver outputs a signal onto a transmission line, the voltage of the initial wave front is defined by a voltage divider using the source voltage, the source impedance, and the transmission line impedance as:Vi=Vs(Z0/Z0+Zs)where Vi is the voltage of the initial wave front, Vs is the voltage of the driver, Z0 is the characteristic impedance of the transmission line, and Zs is the output impedance of the driver.
For example, assume that a driver has a voltage of 5V and an output impedance of 25Ω. In addition, further assume that the transmission line has a characteristic impedance of 75Ω. In this example, the initial voltage of the wave front output from the driver has a magnitude of 3.75V=(5*(75/75+25).
When the input impedance of a receiver connected to the transmission line is different from the characteristic impedance of the transmission line, an impedance discontinuity is present. When an impedance discontinuity exists, some portion of the initial wave front is reflected from the receiver back to the driver. The amount of the initial wave front that is reflected back to the driver is defined by a reflection coefficient as:ρ=Zt−Z0/Zt+Z0 where ρ is the reflection coefficient, Zt is the impedance of the discontinuity, and Z0 is the characteristic impedance of the transmission line.
Thus, when the initial wave front hits a discontinuity at the receiver input, a voltage Viρ is reflected back from the receiver to the driver. As a result, immediately following the reflection, the total magnitude of the voltage on the transmission line at the input of the receiver is Vi+Viρ.
When a receiver input is shorted, the impedance of the discontinuity Zt is equal to zero. As a result, the reflection coefficient ρ is equal to −1 regardless of the value of the characteristic impedance Z0 of the transmission line. In this case, all of the initial wave front is reflected back negatively. As a result, the voltage seen by the input of the receiver is equal to Vi−Vi, or zero, immediately after the wave front has been reflected.
When a receiver input appears to be an open load, the impedance of the discontinuity Zt is equal to infinity. As a result, the reflection coefficient ρ is equal to +1 regardless of the value of the characteristic impedance Z0 of the transmission line. In this case, when an open load is present, all of the initial wave front is reflected back positively. As a result, the voltage seen by the input of the receiver is equal to Vi+Vi, or 2 Vi, immediately after the wave front has been reflected.
In many cases, however, the receiver can not handle a voltage of 2 Vi. In the above example, if the initial wave front has a voltage of 3.75V, then the input of the receiver will see a voltage of 7.5V immediately following the reflection. As a result, there is a need for a circuit that prevents the receiver from experiencing a large voltage due to a reflected wave front.